A conventional solid electrolytic capacitor having a small size and a large capacitance is disclosed in Japanese Patent Laid-Open Publication No. 2000-49048. The capacitor includes a capacitor element having a prismatic shape and including an anode stick having an end thereof embedded in the element. The capacitor element may be manufactured by the following processes. An anode oxide layer is provided on a sintered pellet made, a core member, of valve metal powder material (tantalum powder), and then, a solid electrolyte layer of, e.g., manganese dioxide is provided on the anode oxide layer. A collector layer composed of stratified carbon and silver is provided on the solid electrolyte layer. The capacitor element is then joined with a positive electrode and a negative electrode and then enclosed together in a case. Portions of the case are removed to expose the positive electrode and the negative electrode. The case has both ends solder-plated to form terminals electrically connected with the positive electrode and the negative electrode, respectively.
Another conventional solid electrolytic capacitor is disclosed in Japanese Patent Laid-Open Publication No. 2000-68158. This capacitor includes a capacitor element having a sheet shape. A valve metal base having a dielectric oxide layer provided thereon has an end to provide an anode portion. The dielectric oxide layer is then coated with a solid electrolyte layer excluding the anode portion of the dielectric oxide layer. A collector layer functioning as a cathode portion is provided on the solid electrolyte layer, thus providing the capacitor element. The anode portion and the cathode portion have respective ends soldered or bonded with electrically conductive adhesives to lead frames. The lead frames and the capacitor elements are then coated with an outer coating, so that the lead frames are partially exposed at both ends of the outer coating. The both ends are solder plated to provide terminals electrically connected to a the anode portion and the cathode portion, respectively.
The solid electrolytic capacitor having a multi-layer structure disclosed in Publication No. 2000-68158 may be manufactured by the following processes. Plural capacitor elements having sheet shapes are stacked and fixed so that their anode portions aligned in one direction and joined to the lead frame for the positive terminal. Similarly, the cathode portions of the capacitor elements are bonded to the lead frame for the negative terminal with an electrically conductive adhesive layer, while the cathode portions flare in a direction to the anode portions, thus providing a capacitor body a multi-layer structure. The capacitor body is then enclosed in a resin coating to provide the multi-layer solid electrolytic capacitor.
The above, conventional solid electrolytic capacitors include their positive electrodes and their negative electrodes made of material different from valve metal, as provided for anode. These structures reduce volumetric sizes for providing their capacitances and increase the distances between the anode portions of valve metal to the positive electrodes. Accordingly, the capacitors have their impedances reduced for operating at high frequencies.
A solid electrolytic capacitor may have a large capacitance and a small impedance at high frequencies upon including an anode base of valve metal having an end exposed from a case to function the end as an anode portion serving as a terminal.
However, the valve metal, such as aluminum foil, may easily be coated with a thin oxide layer known as a passive layer. The solid electrolytic capacitor having the aluminum foil may hard has a small impedance since the passive layer functions as a resistor. The aluminum foil may be hardly bonded with plated regions, and thus, may have the bonding strength between the anode base and the plated regions smaller than the conventional capacitors.
The passive layer of the aluminum foil may be washed out with acid or alkali solution. The passive layer is easily developed under the atmosphere or any oxygen environment. Therefore, the passive layer can be hardly removed in either the atmosphere or solution including oxygen dissolved therein. The aluminum foil can be hardly soldered with solder (including solder without lead).